Because of the outstanding electrical characteristics, hardness, contact and corrosion resistances, stability and other desirable properties exhibited by palladium, it is advantageously used as a plating for electrical connectors, printed circuits, and the like, as well as for numerous other industrial and commercial applications. As a substitute for gold and platinum, palladium represents a more economical alternative, both on a simple weight basis and also in terms of covering power, due to its lower density; alloys of palladium, and especially with nickel, have been used in certain applications in order to further reduce costs.
Electroplating baths presently used for producing palladium and palladium alloy deposits typically operate at high alkaline pH values (e.g., 9 to 10) and at relatively high temperatures (e.g., 60.degree. Centigrade), and they commonly contain ammonia as a complexing agent. A disadvantage of using free ammonia is that it must be added constantly, to compensate for evaporation so as to maintain the desired pH, which otherwise tends to drift downwardly during operation; at pH values below about 7.5 or 8 the solubility of certain constituents decreases to the extent that precipitation becomes problematic. The constant ammonia demand, however, makes maintenance of the bath time-consuming, inconvenient, and expensive; in addition, the toxicity of ammonia gas imposes stringent venting requirements, to maintain a safe and pleasant environment.
The prior art has proposed a wide variety of formulations for producing deposits of palladium and its alloys upon workpieces. The following patents (which are, except where indicated, of the U.S.) are exemplary:
Altman U.S. Pat. No. 543,824 provides an electrolytic bath for depositing alloys of platinum and nickel, which purports to contain ethylamine or any other suitable organic base containing nitrogen; methylamine and pyridine are specifically mentioned. The patent suggests addition of the nickel as its ammonium sulfate salt, and evidently the bath contains no free ammonia.
U.S. Pat. No. 1,981,715, to Atkinson, discloses a process for electroplating palladium and its alloys, utilizing a catholyte containing an ammoniacal solution of the metal to be deposited. It is disclosed that organic amines, such as pyridine and ethylenediamine, can be used instead of ammonia to form the desired palladium ammino compounds.
Lambrose U.S. Pat. No. 2,452,308 teaches electroplating palladium from a cyanide solution.
In accordance with Hayashi et al U.S. Pat. No. 3,458,409, palladium is electroplated from an ammoniacal solution containing, as a brightening agent, pyridine, pyridine carboxylic acid or pyridine carboxylic acid amine.
The inclusion of ammonium sulfamate in a palladium plating bath is disclosed in Ickenham U.S. Pat. No. 3,530,050.
Sodium 1,3,6-naphthalenetrisulfonate is utilized, in accordance with Yamamura U.S. Pat. No. 3,580,820, as a brightener for palladium/nickel plating from an ammoniacal solution; aromatic sulfonamides, such as saccharin, are also taught. Yamamura et al U.S. Pat. No. 3,677,909 contains similar disclosure.
Skomoroski et al U.S. Pat. No. 3,925,170 provides a composition for palladium electroplating, which may include ammonium sulfamate.
Sulfite-containing baths for the electrodeposition of palladium and its alloys, which operate at a pH on the alkaline side, are taught in Henzi et al U.S. Pat. No. 3,933,602.
Nobel et al U.S. Pat. No. 3,972,787 utilizes water-soluble quaterinized pyridine salts in baths for electroplating palladium.
Compositions for palladium electrodeposition are provided by Carricchio, Jr. et al U.S. Pat. No. 4,076,599, which contain palladosammine chloride, ammonium sulfamate and an alkali metal sulfite, in addition to ammonium chloride and ammonium hydroxide. A similar bath, used for producing a palladium/nickel alloy deposit, is disclosed in Carricchio, Jr. et al U.S. Pat. No. 4,100,039, wherein the nickel may be supplied as the sulfamate, chloride or sulfate salt.
Ammonia-free palladium baths are taught in Schuster et al U.S. Pat. No. 4,144,141, wherein aminoacetic acid is utilized as a complexing agent. Similar disclosure is found in Heppner et al U.S. Pat. No. 4,242,180.
Ammonium sulfamate-containing electrolytes for palladium electroplating are taught in U.S. Pat. Nos. 4,297,177 and 4,297,179, to Fletcher et al and Skomoroski, respectively; the former discloses the incorporation of saccharin.
Ehrich et al U.S. Pat. No. 4,299,672 provides a bath and process for the galvanic separation of palladium/nickel alloys, in which aliphatic amines, preferably containing as many as six amino groups in a carbon chain of up to eight carbon atoms, are utilized as sequestering agents; specific compounds employed include acids, such as glycine, and polyfunctional amine compounds.
Schulze-Berge U.S. Pat. No. 4,416,740 teaches the use of salts of naphthalene sulfonic acid and aromatic sulfonamides in baths for plating palladium/nickel alloys; brightness and mechanical properties of the coating are improved by the addition of one, or preferably more, acetylene amines and acetylene amino alcohols.
A palladium/nickel alloy electroplating solution is provided by Kanai et al U.S. Pat. No. 4,428,820, which contains ammonium nickel sulfate and tetramminepalladous chloride.
Organosulfonic acids are employed, in accordance with Nobel U.S. Pat. No. 4,465,563, for the electrodeposition of palladium/silver alloys.
Palladium ammine hydroxide is employed by Abys et al U.S. Pat. No. 4,468,296 in an electroplating process, and certain aliphatic polyamine complexes are employed in the palladium plating procedure of Abys et al U.S. Pat. No. 4,486,274.
White deposits of palladium are produced in accordance with Miscioscio et al U.S. Pat. No. 4,487,665 by use of an electroplating bath containing palladosammine chloride, ammonium sulfate and a brightener such as saccharin; potassium chloride may also be incorporated, and small amounts of nickel sulfate may be utilized as a brightener.
In accordance with Henning et al U.S. Pat. No. 4,564,426, palladium/nickel alloy is electroplated from a mildly alkaline bath containing palladosammine chloride, nickel ion, ammonium sulfate, ammonium chloride and ammonium hydroxide.
A "faintly ammoniacal" electroplating bath for the deposition of palladium is provided by Miles United Kingdom patent No. 958,685, which includes sulfamate ions and ammonium ions and has a pH of 5 to 9.
Despite the foregoing, a demand exists for an electroplating bath, and for a method utilizing it, from which a sound and ductile deposit of palladium can be produced, and which operates without liberating free ammonia. It is therefore the broad object of the present invention to provide such a novel bath and method.
It is also an object of the invention to provide a novel bath and method having the foregoing characteristics, which are effective over a broad range of current densities and at low temperatures, to produce matte or bright deposits of high quality.